A number of approaches have been proposed for chip-to-chip and board-to-board optical interconnects, including: (1) channel waveguides; (2) free-space interconnects; and (3) through-substrate optical interconnects. Typical channel waveguide approaches featuring characteristic channel waveguides having cross-section dimensions ranging from 10 to 50 μm have been used to connect transmitters and receivers. Although channel waveguiding technologies are well established, there remain drawbacks including significant propagation losses of the waveguides and tilted mirrors for light coupling from light sources to waveguides, as well as from waveguides to detectors. Additional propagation losses of about 10 dB and higher may result from the fabrication of waveguides longer than a typical wafer size (<20 cm).
Conventional free-space interconnects approaches may result in lower propagation losses. However, such approaches create unacceptable alignment tolerances and reliability issues due to long distance air transmission. Using known through-substrate interconnect approaches, the light beam is launched in the substrate from the top surface such that it propagates along the substrate by bouncing between the top and bottom surfaces. Using these approaches, short distance propagation of less than 1 cm has been demonstrated. On the other hand, the beam collimation, scalability and alignment of refractive, reflective and diffractive elements are problematic for longer distances.